A motor rotor consists of a rotor core, which is made up of a stack of thin steel sheets, each having a through hole in the center formed by punch pressing, a shaft inserted into the shaft bore of the rotor core and securely press-fitted thereto, and magnets mounted in the rotor core.
As one example of a conventional producing method for securely joining the rotor core and the shaft, a fastening structure for the rotor core and the shaft of a motor is disclosed in Patent Document 1. FIG. 14 shows a perspective view of the rotor core disclosed in Patent Document 1. FIG. 15 shows part of a thin steel sheet forming the rotor core as viewed from the direction of an arrow C in FIG. 14. FIG. 16 is a diagram illustrating a condition where a motor shaft has been inserted into the through hole in the rotor core as viewed from the direction of the arrow C in FIG. 14.
According to Patent Document 1, as shown in FIG. 14 and FIG. 15, a through hole 312H in a thin steel sheet 312 is formed with circumferentially equally spaced and alternately arranged indentations 303 and protrusions 304 having different radial lengths. The rotor core 310 is made up of a plurality of such thin steel sheets 312 stacked upon one another such that the indentations 303 of one thin steel sheet 312 are located at the same phase positions of the protrusions 304 of adjacent thin steel sheets 312.
The motor shaft 320 has uneven portions 324 formed by knurling on the outer surface and is inserted into a press-fit hole 311H of the rotor core 310, which is formed by the through holes 312H (with indentations 303 and protrusions 304) of the stack of thin steel sheets 312. According to Patent Document 1, as shown in FIG. 16, when press-fitted, because of the indentations 303 and protrusions 304 of the thin steel sheets 312, the motor shaft 320 has contact portions and non-contact portions as the uneven portions 324 make discontinuous contact with the sheets. According to Patent Document 1, as shown in FIG. 16, each thin steel sheet 312 is allowed to freely deform and deflect toward the axial center AX of the rotor core 310 when the motor shaft 320 is press-fitted into the rotor core 310, so that no stress remains in the rotor core 310 or the motor shaft 320 and that there will be no scratches or burrs after the press-fitting of the motor shaft 320.
FIG. 17 shows another example of a conventional producing method for securely joining a rotor core and a shaft similarly to Patent Document 1 (no particular patent document cited). In this conventional producing method, a rotor core 410 or a stack of thin steel sheets 412 is retained and fastened with a predetermined clamping force, with plate-like jigs 431 and 432 that make surface contact with both end faces 411a and 411b of a lamination stack 411 in the thickness direction HT. The rotor core 410 is heated in this state by high-frequency quenching or the like, and a shaft 420 is inserted into a shaft bore 411H, which has been somewhat enlarged by thermal expansion, after which the rotor core 410 is cooled, so that the rotor core 410 is securely joined to the shaft 420 by shrink fitting.